The present invention relates generally to object tracking systems, and specifically to non-contact, electromagnetic methods and devices for tracking the position and orientation of one or more multiple objects.
Non-contact electromagnetic tracking systems are well known in the art, with a wide range of applications.
Non-contact tracking systems have been described based on electromagnetic, optical and sonographic detection methods. In many applications, electromagnetic tracking is preferred, because unlike other methods, there need not be a clear line of sight between the object and the detectors. Electromagnetic tracking can also be made relatively immune to interference from background signals, thus avoiding problems such as those caused by stray light in optical systems. Furthermore, the components required for electromagnetic tracking are easily produced and inexpensive.
Exemplary prior art patents for electromagnetic tracking systems include U.S. Pat. Nos. 3,046,228 and 3,121,228, which describe a tracking system for measuring the position of an object in a plane. This system uses two orthogonal coils to generate a rotating dipole field in the vicinity of the object, thereby causing electrical currents to flow in two orthogonal sensor coils. Measurement of the sensor coil current signal is used to determine the object""s position.
U.S. Pat. Nos. 3,868,565, 3,983,474 and 4,017,858 describe a tracking system using three orthogonal coils to generate a dipole field, which nutates around a direction vector. The object""s position is determined by measuring the current signals generated in three orthogonal sensor coils.
U.S. Pat. No. 4,054,881 describes a tracking system using three coils to generate electromagnetic fields in the vicinity of the object. The fields generated by these three coils are distinguished from one another by open loop multiplexing of time, frequency or phase. The signal currents flowing in three orthogonal sensor coils are used to determine the object""s position, based on an iterative method of computation.
U.S. Pat. No. 4,314,251 describes a variation on the ""881 patent, wherein only five coils are needed in total, and a non-iterative method of computing is used.
U.S. Pat. No. 4,287,809 describes a tracking system with two or more coils to generate electromagnetic fields, using time multiplexing to distinguish between them. Three orthogonal sensor coils are used to determine the object""s position.
Finally, U.S. Pat. No. 4,849,692 describes a tracking system with three orthogonal coils to generate periodic DC electromagnetic fields. A receiver measures the DC field at the object to determine its position.
It would be desirable to provide an electromagnetic tracking method whereby the object or objects being tracked have no physical connection to the surroundings. Desirably, such a method would require that only passive components be mounted on the object, and the object""s motion would be unrestricted within the field of detection. All power, active components and additional signals conveyed from the transmitter to the sensor and signal processing circuits would preferably be restricted to the fixed portion of the tracking system.
The present invention seeks to provide a new, non-contact method for tracking the six-dimensional position and orientation of an object, which may move and rotate relative to an external frame of reference (translation along and rotation about three orthogonal axes) within a region of interest. The method may be used to track a single object or multiple objects simultaneously in a single system.
In a preferred embodiment of the present invention a transponder is mounted on or inside the object to be tracked. One or more electromagnetic field generators, which are fixed in the external reference frame, generate electromagnetic fields in the vicinity of the object. The field generators"" electromagnetic fields cause the transponder to generate electromagnetic signals, which are detected by one or more electromagnetic sensors, which are fixed in the external reference frame. Sensor circuits are provided, which determine the six-dimensional position and orientation of the transponder in relation to the external reference frame, using the signals received by the sensors.
In a preferred embodiment of the present invention, the transponder comprises three coils, which define three independent axes and are preferably orthogonal. The transponder coils are coupled to respective electrical circuits, which generate electromagnetic signals with different, characteristic frequencies when excited.
Furthermore, in a preferred embodiment of the present invention, the three coils in the transponder are wound around a common center preferably a ferromagnetic core. It will be appreciated, however, that the axes of the transponder may be defined by other means, such as antennae, that are capable of receiving and transmitting electromagnetic fields with a preferred direction. Other aspects of the present invention described herein in relation to transponder coils may equally be applied to transponder antennae of other types.
In a preferred embodiment of the present invention, the electrical circuits in the transponder are passive resonant circuits. These resonant circuits may be formed by connecting each of the transponder coils to a respective capacitor, the capacitors being chosen so that each of the circuits thus formed has a different, known resonant frequency. The electromagnetic field generators generate time-varying fields, having components at the resonant frequencies of the transponder circuits, which cause these circuits to resonate. The signal generated by each of the coils may then be identified and distinguished from other signals by its respective resonant frequency.
Another preferred embodiment of the present invention provides that multiple objects, each with its own transponder, may be tracked simultaneously without confusion, providing that every one of the coils in the various transponders to be tracked is coupled to an electrical circuit having a unique, known resonant frequency.
In a preferred embodiment of the present invention, the electromagnetic field generators and sensors comprise coils which are coupled to appropriate electrical source and sensor circuits, respectively. More generally, however, field generation and/or sensing may be accomplished by other types of antennae, which are known in the art. Other aspects of the present invention described herein in relation to field generator and sensor coils may equally be applied to field generator and sensor antennae of other types.
Preferred embodiments of the present invention provide that a total of only four field generator and sensor coils be used. One such embodiment uses three field generator coils and one sensor coil; another such embodiment uses one field generator coil and three sensor coils; and still another such embodiment uses two field generator coils and two sensor coils. Such embodiments can be used to track six degrees of motion of one or multiple objects. Other preferred embodiments may have a total of five, six or more field generator and sensor coils. Where more than four sensors are used, the additional sensors provide redundancy and improved signal-to-noise ratio.
In a preferred embodiment of the invention, one or more coils may function as both electromagnetic field generators and sensors. In this embodiment, both source and sensor circuits are coupled to the coil. Source circuits provide an electrical pulse to the coil, or else the source electrical current to the coil is rapidly switched off or on. Sensor circuits then detect the electromagnetic signals that are subsequently generated by the transponder and received by the sensors.
In preferred embodiments of the present invention, field generator and sensor coils or antennae are formed of different coils, preferably constructed according to prescribed geometrical criteria, so that the signal levels received by the sensor coils remain within a limited, prescribed range, independent of the position of the transponder within the region of interest. Such embodiments permit the sensor circuits to determine the position and orientation of the transponders with equal accuracy and without ambiguity throughout the region of interest.
Source circuits used to drive the field generators may provide continuous or pulsed alternating current signals at the resonant frequencies of the transponders. The sensor circuits then receive signals from both the field generators and the transponders.
These signals can be easily distinguished, however, since they are 90xc2x0 out of phase. It should be understood in this regard that a single field generator may be driven by a multi-frequency signal.
One aspect of preferred embodiments of the present invention, having two or three field generator coils, is that the signal processing and computation circuits are capable of distinguishing among the signals generated by the transponder coils in response to the respective field generator coils. Where pulsed sources are used, each of the field generator coils is coupled to a respective pulse generating circuit, and these circuits are mutually timed so that pulses are applied to each of the coils in sequence, at known, mutually exclusive, times. The sensor circuits are synchronized with the times of the pulses, so that they receive and process signals generated by the transponders synchronously with the field generator pulses. The sensor circuits thus differentiate between signals received from the transponders according to the sequence of applying pulses to the respective field generator coils. In order to allow for more continuous monitoring, however, in a preferred embodiment of the invention, only one field generator is used and at least three sensors are used.
In accordance with preferred embodiments of the present invention, the sensor circuits comprise signal processing and computation circuits, coupled to the sensor coils, which determine the three-dimensional position and three-dimensional rotational orientation of the transponder or transponders, based on the signals received by the sensor coils. The signals received by the sensor coils are typically amplified and then separated into their respective frequency components, by filtering methods known in the distinguish between the respective signal amplitudes (and phases, where appropriate) received from the transponder coils.
A preferred embodiment of the present invention further provides computational circuitry, which applies one or more matrix transformations to the measured signal amplitudes so as to compute the position and/or rotational orientation of the transponder. Other mathematical operations, which are known per se in the art, may also be used to compute the transponder""s position and/or orientation.
The transponders of the present invention may also be attached to game pieces or toy figures for use in interactive computer games, such as those described in PCT patent application number PCT/US95/10096, filed Jul. 25, 1995, and titled xe2x80x9cComputerized Game Board,xe2x80x9d which is incorporated herein by reference.
Transponders in accordance with the present invention may also be fastened to a person""s hand or to movable control objects, for use in actuating remote systems by sensing the motion of the hand or control objects.
The present invention may be usefully applied to cockpit avionics systems, in which the position of the pilot""s helmet is tracked in relation to the cockpit in order to appropriately adjust a helmet-mounted display or weapons sighting apparatus.
In a similar application, the transponder of the present invention may be coupled to a helmet or other head-mounted apparatus to provide a determination of the position and orientation of the head for use in virtual reality displays, simulation training and games.
It will be appreciated that these applications are presented here only by way of example and that the present invention will be useful in a wide range of other applications.